This invention relates to elastic polymer consisting essentially of units derived from propylene.
Both crystalline and amorphous polypropylenes are well known. Crystalline polypropylene is generally regarded as consisting at least prevailingly of the isotactic or syndiotactic structure and amorphous polypropylene is generally regarded as consisting at least prevailingly of the atactic structure. U.S. Pat. Nos. 3,112,300 and 3,112,301, both to Natta et al., describe isotactic and prevailingly isotactic polypropylene, respectively. Structural formulae for isotactic and syndiotactic polypropylene are given in U.S. Pat. No. 3,511,824 to Listner. "Atactic" polypropylene is defined as polypropylene in which the substituent methyl groups are arranged randomly above and below the backbone chain of atoms when the latter are all in the same place.
Most commercial grades of polypropylene are highly crystalline and, as is well known, are used in the manufacture of plastic products. Amorphous polypropylenes are also available commercially and are generally gummy materials of little strength. The amorphous polypropylenes are usually present as a small fraction in prevailingly isotactic polypropylene and can be readily extracted. They are normally used in adhesive applications.
Rubbery polypropylenes are also known. Such products have been said to be produced directly by conventional polymerization using particular catalysts, by repeated extractions of conventional polypropylene, by chemical treatment of crystalline polypropylene and by sequential polymerization processes. Representative rubbery polypropylenes are described in U.S. Pat. Nos. 3,329,741 to Schrage et al., 3,175,999 to Natta et al., 3,511,824 to Listner and 3,784,502 to Gobran et al. Such polypropylenes, however, have not found significant use in products requiring an elastic polymer.
There is a need, therefore, for a polypropylene with practical elastic properties that can be produced as a direct reaction product by a practical process.